Electrochemical response of zirconia-coated 316L stainless-steel in a simulated proton exchange membrane fuel cell environment

Article abstract of DOI:10.1016/j.jallcom.2008.06.093

The corrosion resistance of zirconia-coated austenitic stainless-steel 316L was investigated in a simulated proton exchange membrane fuel cell (PEMFC) environment. The zirconia coating was performed using a sol-gel dip coating method and electrochemical t

Full text of DOI:10.1016/j.jallcom.2008.06.093

Journal of Alloys and Compounds 474 (2009) 268–272
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrochemical response of zirconia-coated 316L stainless-steel in a simulated
proton exchange membrane fuel cell environment
W.G. Lee, K.H. Cho, S.B. Lee, S.B. Park, H. Jang∗
Department of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2008
Received in revised form 2 June 2008
Accepted 15 June 2008
Available online 3 August 2008
The corrosion resistance of zirconia-coated austenitic stainless-steel 316L was investigated in a simulated
proton exchange membrane fuel cell (PEMFC) environment. The zirconia coating was performed using a
sol–gel dip coating method and electrochemical tests were carried out at 80 ◦C in 1 M H2SO4 solution to
accelerate corrosion. The results showed that the precursor containing zirconium alkoxide and zirconium
acetate hydroxide changed into tetragonal zirconia, producing a surface film without cracks. Potentiody-
namic tests showed that the corrosion resistance of the zirconia-coated 316L stainless-steel was improved
by one order of magnitude compared to the bare specimen in terms of the current density. Potentiostatic
experiments also showed enhanced corrosion prevention due to zirconia coating under the simulated
PEMFC conditions. However, the high interfacial contact resistance (ICR) of the zirconia film suggested
that the modification of the zirconia film should be performed to impart higher electrical conductivity for
the successful application of zirconia-coated metallic bipolar plates.
Keywords:
Sol–gel
Zirconia
Proton exchange membrane fuel cell
Corrosion resistance
Interfacial contact resistance
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Metallic bipolar plates; however, suffer from low corrosion resis-
tance compared to graphite and carbon composites as a result of
The bipolar plate components for supporting the membrane
electrode assembly (MEA) are considered one of the main chal-
lenges hindering the success of fuel cells. Currently, graphite bipolar
plates are widely used for a commercial proton exchange mem-
brane fuel cell (PEMFC) assembly, since graphite is electrically
conductive, resistance to corrosion and easy to machine. However,
the plates produced using graphite have to be more than a few mil-
limeters thick to support the MEA, as they are brittle and porous
and; therefore, are responsible for a major proportion of the weight
and volume of a stack.
the possible acid leaching from sulphonation of the membrane
electrolyte assemblies, which is detrimental to the durability of a
PEMFC. To investigate the corrosion mechanism of metallic bipo-
lar plates under a PEMFC operating environment, the susceptibility
of various metals, such as titanium, aluminum and stainless-steel,
to corrosion has been studied in a simulated environment [2–7].
Surface treatments, using electroplating, plasma coating, chemi-
cal vapor deposition (CVD), and physical vapor deposition (PVD),
have also been carried out to prevent corrosion of the metal surface
[8,9].
Several candidates, bipolar plates based on metals and compos-
ite materials, have been developed to replace the graphite plate
in attempts at reducing the stack size and extending the service
life. Of these PEMFC bipolar plate materials, metals and alloys have
recently attracted much attention due to their good thermal and
electrical conductivity and easy fabrication via stamping or etch-
ing to produce channels for reactant and product gases. Another
advantage of metal plates is their low gas permeability, which
allows the use of thin plates to reduce the stack volume [1–5].
Various surface treatments have been performed using the
sol–gel method to produce ceramic coatings on various substrates,
such as dip-coating, spray coating, spin coating and electrophore-
sis, as this has advantages over other traditional methods in terms
of equipment, processing and production costs. Another benefit of
employing the sol–gel method is the good adherence of the film
to both metallic and non-metallic substrates [10–16], providing
excellent protection from oxidation and acid attack [10,17,18].
In the present work, the main focus was on improving the cor-
rosion resistance of zirconia-coated 316L stainless-steel, as zirconia
has good corrosion resistance and a thermal expansion coeffi-
cient close to various metals. The zirconia-coated steel surface
was produced by dip coating followed by calcination at 800 ◦C.
Electrochemical tests were performed under a PEMFC operating
environment to examine the corrosion resistance of the zirconia
∗
Corresponding author at: Department of Advanced Materials Engineering, Korea
University, 1, 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea.
Tel.: +82 2 3290 3276; fax: +82 2 928 3584.
E-mail address: hojang@korea.ac.kr (H. Jang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.06.093

W.G. Lee et al. / Journal of Alloys and Compounds 474 (2009) 268–272
269
Table 1
Chemical composition of 316L stainless-steel (wt.%)
Cr
Ni
Mo
Mn
S
Si
C
P
Fe
16.00–18.00
10.00–14.00
2.00–3.00
2.00↓
0.030↓
0.75↓
0.03↓
0.045↓
Balance
layers on the 316L stainless-steel substrate. The interfacial con-
tact resistance, which is a critical parameter for bipolar plates in
a PEMFC, was also analyzed.
2. Experimental details
A commercially available 316L austenitic stainless-steel sheet was used as the
substrate for the surface treatment using a sol–gel method. The composition of the
steel is given in Table 1. The sheet was 2 mm in thickness, and cut into a 16 mm
diameter for the sol–gel coating and subsequent corrosion tests. Before coating,
the specimens were polished with #2000 SiC abrasive papers and degreased with
acetone using an ultrasonic cleaner.
Zirconium alkoxide (zirconium acetate hydroxide (CH3CO2)xZr(OH)y,
Sigma–Aldrich) was diluted in ethanol (C2H5OH) and used as the zirconia
source. The solution was mixed with acetone (CH3COCH3) to prevent the formation
of zirconium hydroxide [19,20]. Acrylic acid (C3H4O2) was added to the solution to
assist the formation of tetragonal zirconia [21,22]. The solution was homogenized
with a magnetic stirrer for 5 min at room temperature, with the final solution
having a pH of about 3.4.
The 316L stainless-steel substrate was dipped into the solution, withdrawn at
5 cm/s and dried at room temperature for 1 h. The samples were calcined for 1 h in a
quartz-tube furnace at 800 ◦C under an argon atmosphere. The structure of the zirco-
nia thin film was examined by X-ray diffraction analysis (Rigaku D/MAX-2500V/PC)
using Cu K␣ radiation (ꢀ = 1.54 Å). The surface morphology of the film was examined
using scanning electron microscopy (SEM, Hitachi S-4300) and the composition was
analyzed using auger electron spectroscopy (AES, PHI 680) and X-ray photoelectron
spectroscopy (XPS, VG Microtech ESCA2000h). The high resolution image of the zir-
conia film was examined using transmission electron microscopy (FEI Technai G2
F30).
Potentiodynamic and potentiostatic tests were carried out using a computerized
potentiostat/galvanostat (VMP3-CHAZ, Princeton Applied Research) to evaluate the
electrochemical response of the 316L stainless-steel both before and after the zir-
conia coating. The corrosion rate of the stainless-steel was accelerated using a 1-M
H2SO4 solution under PEMFC operating conditions at 80 ◦C. The corrosion circuit
used in this study consisted of three electrodes; a graphite counter electrode, a sat-
urated calomel electrode as the reference electrode and a stainless-steel specimen
as the working electrode.
Fig. 2. SEM micrograph of zirconia-coated 316L stainless-steel.
graph (Fig. 2). The zirconia film showed surface traces produced
from the final polishing step using abrasive papers, while the
surface roughness (Ra) was decreased from 0.202 to 0.032 ␮m,
but no cracks or pores were observed. No indication of surface
cracks in the figure suggests that the film was thin enough to
prevent crack initiation as a result of stress build-up at the het-
erophase interface due to a lattice mismatch or the dissimilar
thermal expansion coefficients [16,23,24]. The crystal structure
of the zirconia film on the stainless-steel substrate was exam-
ined by XRD (Fig. 3), the profile of which showed two distinct
peaks, with d values at 0.208 and 0.17 nm from the austenitic
stainless-steel phase, and an additional peak at 0.2995 nm from the
tetragonal zirconia phase [10,25]. The grain size of the zirconia film
was also investigated using high resolution transmission electron
microscopy (Fig. 4). The high resolution micrograph of the zirco-
nia film showed grains with diameters in the size range 5–10 nm.
It is well known that, after high temperature annealing, the zir-
conia nanoparticles at room temperatures exhibit the tetragonal
structure when the crystallite size is less than 30 nm in diameter
[22,26,27].
The interfacial contact resistance (ICR) was obtained from the voltage drop
across the specimen by applying an electrical current (1 A) via two copper plates
(Fig. 1) [6] in the force range 5–50 kgf. ICR was monitored using an Agilent 34401A
potentiometer.
3. Results and discussion
3.1. Microstructure and composition of zirconia film
The structure and surface morphology of the zirconia film on
the stainless-steel substrate was examined with an SEM micro-
Fig. 1. A schematic of the device to measure interfacial contact resistance.
Fig. 3. X-ray diffraction pattern of zirconia-coated 316L stainless-steel.

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W.G. Lee et al. / Journal of Alloys and Compounds 474 (2009) 268–272
Fig. 6. X-ray photoelectron spectrum of Zr 3d5/2 and Zr 3d3/2
.
Fig. 4. High resolution image of the zirconia film.
energy of Zr 3d from the zirconia film is known to be attributable
to the oxygen vacancies in the ZrO2 film, which exhibit lower oxi-
dation states (ZrO2−x) [28–30].
The composition of the zirconia film was examined by AES. The
depth profiles in Fig. 5 show the Fe, Cr, Zr, C and O composition as
a function of the sputter time. This figure indicates that the thick-
ness of the film was less than a micron, as the sputtering rate of a
standard specimen under the same conditions was approximately
23.8 nm/min. The auger profiles showed high Zr and O concen-
trations on the steel surface, with broad interfacial composition
profiles observed at the boundary between the film and the sub-
strate resulting from the initial roughness (Ra = 0.202 ␮m) of the
steel surface.
In order to confirm the oxidation states of the zirconia, an
XPS spectrum was obtained from the zirconia film (Fig. 6), which
showed Zr 3d5/2 and Zr 3d3/2 spin–orbit split components of Zr 3d at
181.6 and 183.8 eV, which were slightly lower than those from the
fully oxidized zirconium in its Zr4+ state. The slightly lower binding
3.2. Corrosion resistance of zirconia film
The corrosion behavior of the 316L stainless-steel was investi-
gated both before and after zirconia coating using potentiodynamic
tests under PEMFC operating conditions, which measures the cur-
rent as a function of the potential. The polarization curves obtained
from the potentiodynamic test at 80 ◦C in 1 M H2SO4 solution are
shown in Fig. 7. The polarization curves exhibited active, passive
and trans-passive states of corrosion. This figure indicates that the
current density of the zirconia-coated stainless-steel was one order
of magnitude lower than that of the bare stainless-steel, indicating
an improvement in the corrosion resistance after zirconia coating.
From the corrosion behavior of the 316L stainless-steel speci-
mens both before and after the coating, the polarization resistance
Fig. 5. Composition profiles of the zirconia-coated 316L stainless-steel surface
obtained from AES depth profiling.
Fig. 7. Potentiodynamic curves of bare 316L stainless-steel and 316L stainless-steel
coated with zirconia measured at 80 ◦C in 1 M H2SO4 solution.

W.G. Lee et al. / Journal of Alloys and Compounds 474 (2009) 268–272
271
Table 2
Polarization resistance (Rp) and Tafel constants of bare 316L stainless-steel and 316L stainless-steel coated with zirconia
Ecorr (mV)
Icorr (␮A/cm2)
ˇa (mV)
ˇc (mV)
Rp (ꢁ/cm2)
Bare 316L stainless-steel
316L stainless-steel coated with zirconia
−252.3
−280.2
58.678
0.800
12.0
35.2
37.9
65.4
67.5
12436.7
at 150 N/cm2, which is the normal compaction force applied for
the attachment of bipolar plates in a commercial stack, were 104
and 275 mꢁ cm2 for the bare and zirconia-coated stainless-steel
plates, respectively. Of particular note was the high contact resis-
tance of the nanograin zirconia film on the metal substrate, as the
thin nanocrystalline zirconia film normally shows higher electrical
conductivity than microcrystalline zirconia due to grain boundary
effects [33], suggesting further investigations on the modification
of the zirconia composition by doping conducting constituents into
the zirconia film to improve their electrical conductivity [34–36].
(Rp) was calculated using the linear polarization method, which
was found to be inversely proportional to the corrosion rate, as
obtained from the equation; Rp = ˇaˇc/2.3icorr(ˇa + ˇc) [31], where
ˇa and ˇc are the cathodic and anodic Tafel constants (activation
polarization), respectively, and icorr the corrosion current density.
The corrosion parameters obtained from the polarization curves in
Fig. 7 are shown in Table 2. The corrosion rate (icorr) of the coated
316L stainless-steel was approximately 70 times lower than that
of bare stainless-steel under PEMFC conditions. The polarization
resistance of the coated 316L stainless-steel was also enhanced
about 180 times.
Potentiostatic tests were also carried out to examine the corro-
sion resistance of the zirconia-coated stainless-steel by monitoring
the current density at the anode and the cathode under a PEMFC
operation environment. The PEMFC working conditions were sim-
ulated by operating the anode and cathode at −0.1 V (vs. SCE) and
at 0.6 V (vs. SCE), respectively. During the operation, the anode
and cathode were aerated with hydrogen gas and oxygen gas,
respectively. Fig. 8 shows the current density measured from the
zirconia-coated stainless-steel as a function of time. The current
density from both the anode and cathode showed no noticeable
changes during the potentiostatic tests, despite the accelerated
corrosion condition in 1 M sulfuric acid solution, suggesting that
the zirconia layer effectively prevented the stainless-steel sub-
strate from corrosion [32]. The surface morphology of the bare and
zirconia-coated specimens were examined after the potentiostatic
tests using SEM (Fig. 9). The SEM micrograph of the zirconia-coated
specimen showed no pits after the corrosion test; whereas, the bare
stainless-steel did, confirming the improved corrosion resistance
due to the zirconia film produced by the sol–gel method.
3.3. Interfacial contact resistance of zirconia film
High electrical conductivity is one of the most important
requirements for a commercial PEMFC bipolar plate. This is because
a stack is usually comprised of many cells for a certain voltage
to be gained through a series of attached cells. For commercial
PEMFC bipolar plates, the electrical conductivity has to be greater
than 100 S/cm [3], with a recommended ICR of less than 20 mꢁ m2,
which is the typical ICR value for graphite bipolar plates [2]. Var-
ious metals and alloys have been considered as good candidates
for a bipolar plate, since they normally satisfy the requirement for
electrical conductivity. However, the contact resistance of a metal
plate is often greater than the recommended specification due to
surface passivation. In addition, surface treatments of the metal
plate normally further increase the contact resistance [2]. There-
fore, the interfacial contact resistance after surface coating has to
be carefully examined.
The interfacial contact resistances of the bare and zirconia-
coated 316L stainless-steel were measured as a function of the
compaction force (Fig. 10). Fig. 10 shows that the ICR value
decreased with increasing compaction force for both specimens
due to increases of the contact areas, which act as electrical junc-
tions. The figure also shows that the ICR of zirconia-coated 316L
stainless-steel was higher than that of the bare 316L stainless-steel
over the entire compaction force range. The higher ICR value of
coated 316L stainless-steel was attributed to the higher electri-
cal resistance of the zirconia film. The contact resistance values
Fig. 8. Potentiostatic curves of 316L stainless-steel coated with zirconia measured
in 1 M H2SO4 solution at 80 ◦C: (a) O2 purging at 0.6 VSCE and (b) H2 purging at
−0.1 VSCE
.

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W.G. Lee et al. / Journal of Alloys and Compounds 474 (2009) 268–272
The zirconium oxide film produced on the steel surface showed
nanocrystalline tetragonal structure. Potentiodynamic tests
a
showed that the current density of the zirconia-coated stainless-
steel was one order of magnitude lower than that of the bare
stainless-steel, suggesting a much improved corrosion resistance
due to the zirconia coating. Also, the zirconia films effectively pro-
tected the steel substrate during corrosion tests under accelerated
corrosive conditions, i.e. in a 1-M H2SO4 solution at 80 ◦C. How-
ever, despite the thin nanocrystalline structure of the zirconia film,
the interfacial contact resistance of the zirconia-coated steel plate
was two to three times higher than that of the bare specimen, sug-
gesting that modification of the zirconia film would be required to
improve the electrical conductivity of the film.
Acknowledgement
This work was supported by grants from the Seoul R&BD Pro-
gram of the Korea University.
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Zirconium oxide was coated onto the surface of 316L stainless-
steel, using a sol–gel method, to improve the corrosion resistance.